Although the molecular target for drug therapy of sickle cell disease has been known for over 50 years, the only drug approved by the United States Food and Drug Administration is hydroxyurea. Hydroxyurea reduces the probability of vas-occlusion by increasing the synthesis of fetal hemoglobin, which dilutes the abnormal hemoglobin S, markedly slowing its polymerization to form the fibers that distort (sickle) and make the red cells inflexible. This drug is, however, only partially successful in reducing the frequency of pain crises and the chronic organ damage characteristic of the disease. The search for additional and more effective therapeutic agents has been severely hampered by the lack of a high throughput assay for inhibition of sickling. We are developing a method to rapidly, accurately and sensitively test for anti-sickling activity in large populations of human red blood cells by measuring the distribution of sickling times. The first drug screen was carried out on the 2,000 compound library of Microsource Discovery, which contains 800 of the approximately 3,500 FDA-approved drugs. This screen was carried out under hyper-osmolar conditions to shorten the delay time for technical reasons. Now with technical improvements we can make measurements at physiological osmolarity and have rescreened the library. Several candidate drugs appeared in this screen, and we are now studying the concentration and time dependence to determine therapeutic potential. We have also established an hemolysis assay, which is the most likely source of red cell toxicity, as well as the capability to measure blood oxygen binding curves. If the delay time is increased significantly at serum concentrations found for an FDA-approved drug, then, in principle, only two simple additional tests are required before clinical trials could begin, namely the hemolysis test and an oxygen binding curve to insure that the oxygen delivery function of the treated red cells is intact.